The Milky Way's dark secret

Oct 16, 2002

It has long been believed that a supermassive black hole lurks at the centre of the Milky Way galaxy but there has always been a nagging doubt that some other exotic astrophysical object might be responsible for the concentration of mass there instead. Now an international team of astronomers has followed a star for two-thirds of a complete orbit around the centre of our galaxy and ruled out any of the possible alternatives (R Schödel et al. 2002 Nature419 694). They find that the black hole at the centre of the Milky Way weighs 3.7±1.5 million solar masses.

Since black holes do not by definition emit light, their existence has to be inferred some other way. The two most common techniques are to follow the motions of stars around the suspected black hole, or to detect a characteristic pattern of X-rays emitted by material that is heated before being swallowed by the black hole. Astronomers believe that there is a supermassive black hole at the centre of every galaxy in the universe. However, until now they had not been able to rule out the possibility that a dense cluster of dark stellar objects or a ball of massive, degenerate fermions were mimicking the effects of a black hole.

Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching and co-workers elsewhere in Germany, France, Israel and the US followed the orbit of a star around the compact radio source Sagittarius A* - which is thought to surround the supermassive black hole at the centre of the Milky Way - over a ten-year period using a variety of ground-based telescopes.

Their most recent obseravtions - with one of the 8-metre telescopes that make up the European Southern Obsveratory’s Very Large Telescope in Chile - exploited adaptive optics to correct for atmospheric blurring and obtain images that were 20 times sharper than before. The team also used the VBLA array of radio telescopes to improve the accuracy of the observations.

The new data allowed Schödel and co-workers to calculate that the star had a highly elliptical Keplerian orbit with a period of 15.2 years and that it approached within 17 light hours of Sgr A*. By comparison it takes the Sun 230 million years to circle the Milky Way.

The team now hopes to study the motion of even fainter stars near Sgr A* and to probe for the first time various effects predicted by the general theory of relativity.